This invention relates generally to image-forming systems and processes, and specifically to systems and processes involving the transformation of color image signals from one form into another.
Modern imaging systems used in the pre-press, printing, computer graphics, and related industries make extensive use of color image information. This information is sometimes derived from scanning photographs and other "hardcopy" images, and is in other instances obtained directly from a computer-generated graphics file.
Various sources of color images produce digital data files in which color is specified in various ways. Furthermore, different types of output devices, for instance computer monitors, color ink-jet and laser printers, and imagesetters, are designed to operate using various different standards for defining colors.
One common component of an image-forming system is a page description language interpreter, for instance as produced by Adobe Systems Incorporated for interpretation of the PostScript.RTM. page description language. One function of apparatus employing such interpreters is to accept, as input, signals corresponding to imaging commands written in a page description language and to produce, as output, signals recognizable by an imaging engine, such as a laser print engine of a conventional color laser printer. Further pertinent background is presented in the POSTSCRIPT LANGUAGE REFERENCE MANUAL, SECOND EDITION, Adobe Systems Inc., pp. 176-192, 295-302(Addison-Wesley 1990), the contents of which are reference.
In performing the transformation from input to output in such apparatus, it is often necessary to convert signals representing a color from one format to another format. Such formats are sometimes referred to as color spaces. For example, a color produced by a computer graphics workstation may initially be specified as separate Red, Green, and Blue values in an "RGB" color space. For printing of a corresponding image, it may be necessary to transform a color signal from the RGB color space to, for example, the "CMYK" color space for printing using Cyan, Magenta, Yellow, and Black (or "Key") colorants. Intermediate color signal transformations also are often called for in order to provide certain benefits, such as the ability to work with a number of different image source devices and image forming devices.
Two general approaches are conventionally used for such color space transformation. In one approach, a mathematical relationship between the input color space and the output color space is determined, and a computer program is implemented to compute output color space signal values from any given set of input color space signal values.
A second approach is to use a conventional "look-up table" stored in computer memory that, for particular values of input color space signals, provides corresponding values of output color space signals. In some instances, more possible inputs exist than would be practical to provide as look-up table inputs. "Sparse" look-up tables, with interpolation for in-between values, are typically used in such cases.
The processing required to perform such color space transformations using conventional techniques is computationally intensive and requires a relatively large amount of computer memory, which in turn requires the use of more expensive equipment to perform such processing.
It would be desirable for an image processing system to process color transformations in a manner that is more efficient than possible with the known techniques. No known solution adequately addresses the need for a simple, flexible, inexpensive system and process for color transformations.